In fluidized-bed catalytic cracking processes, known in the art as fluid catalytic cracking (FCC) processes, the hydrocarbon feedstock is injected into a reactor in the form of a column in which the catalyst is maintained in suspension and flows either in an essentially ascending stream (in which case the reactor is called a riser) or in a descending stream (in which case the reactor is called a dropper).
For greater clarity, what follows will refer to the case of ascending-flow reactors, but it will be obvious to one skilled in the art that it might just as well refer to descending-flow reactors; and the feedstock injectors which are the object of the present invention should be understood to be applicable to both reactor types.
In a riser, the regenerated catalyst is introduced at the base of the reactor in a fluidized and hot state (at from 650.degree. to 850.degree. C.) at the same time as a suspending gas below the zone of injection of the hydrocarbon feedstock. The latter is introduced into the reactor substantially in the liquid state and at a temperature ranging from 80.degree. to 300.degree. C.
Between the zone of injection of the feedstock and the top of the reactor, the catalyst gives up some of its heat energy to the feedstock, which is then vaporized and cracked into light hydrocarbons. This has the effect of rapidly increasing the volume of the gases, which then transport the catalyst at an accelerated rate to the top of the reactor, where it is separated from the hydrocarbons. At the outlet of the riser, the mixture of hydrocarbons and catalyst particles attains an equilibrium temperature which usually ranges from 470.degree. to 530.degree. C.
In the course of these operations, a small portion of the feedstock (generally from 3 to 12 weight percent) forms a solid hydrocarbon deposit or "coke" on the catalyst particles which reduces the catalytic activity of the catalyst and limits the conversion of the feedstock to upgradable products. The catalyst therefore has to be regenerated by burning off this coke deposit before it is reintroduced into the reactor for a new cracking cycle.
The deposition of coke on the catalyst generally is the more severe the heavier the injected feedstock is and the more difficult it therefore is to vaporize. The unvaporized fractions then form on the catalyst a film composed mainly of coke and heavy hydrocarbons.
With a view to limiting this process, it is therefore important to prevent the liquid fractions of the hydrocarbon feedstock from coming into contact with the catalyst before they are vaporized and then cracked, and it is therefore necessary to reduce the duration of vaporization of the feedstock.
Moreover, it is well known in the art that it is advisable to inject the feedstock to be treated into the reactor at high velocity and in the form of very fine droplets. (See U.S. Pat. No. 2,891,000 and 2,994,659.)
To this end, it has been proposed to employ feedstock injectors comprising a venturi tube. (See French patent 2,102,216, U.S. Pat. Nos. 3,240,253 and 4,523,987, and European patent 157,691.) However, this approach poses many problems when used for injection into fluidized-bed reactors. In fact, the velocities at the level of the throat of the venturi tube give rise to the separation of fluid stream lines from the walls of the injector, the formation of backflow currents, and the penetration of catalyst particles into the injector, resulting in marked erosion of the injector and rapid deterioration of performance.
Impact-type injectors have further been proposed (see U.S. Pat. No. 4,434,049) which employ mechanical means for atomization of the feedstock; however, the quality of atomization of the feedstock is not nearly as good for a given energy input.